Adaptive-rate transmission systems are known in the art of data communications. Modems used in such systems vary their transmission rates depending on the noise conditions of the communication channel. Typically, the transmission rate is determined at the initiation of communications between a pair of modems, using an agreed-upon startup protocol to assess the line conditions and choose the maximum transmission rate that will satisfy a minimal signal-to-noise ratio (SNR) criterion. Equivalently, the rate may be chosen so that the bit error rate (BER) does not exceed a predetermined maximum.
During an extended communication session, noise conditions on the communication channel are likely to change. For example, in a central office (CO) with many modems serving multiple subscriber lines, near-end crosstalk (NEXT) and far-end crosstalk (FEXT) can increase dramatically during a session due to communication activity on other lines. If the noise increase is too great, it will cause the modems to lose synchronization. When this occurs, it is necessary to repeat the startup protocol (referred to as “retraining”) in order to determine a new, lower transmission rate. To avoid this situation, startup protocols typically specify a target signal margin, and the transmission rate is chosen so that the measured SNR exceeds a baseline SNR by at least the specified margin. The “baseline SNR” is generally the SNR for which the modem is expected to operate with the maximum allowed BER. Choosing the appropriate margin involves a tradeoff between reducing the likelihood of synchronization loss when the noise increases, and maximizing the exploitation of available channel bandwidth under current noise conditions.
Adaptive rate transmission with a startup training protocol is used in various types of Digital Subscriber Line (DSL) systems, including Single-pair High-speed DSL (SHDSL), as described in Draft Recommendation G.991.2 of the International Telecommunications Union (ITU), entitled “Single-Pair High-Speed Digital Subscriber Line (SHDSL) Transceivers” (January, 2001), which is incorporated herein by reference. As described in the Draft Recommendation, the central office modem (STU-C) and the customer premises modem (STU-R) carry out a power measurement modulation session (PMMS) at communication startup in order to decide on the communication rate. The target margin is fixed in this Draft Recommendation.
Moss et al. propose a standard procedure for specifying the target margin parameter for SHDSL in ITU Temporary Document CF-042, entitled “G.shdsl: Proposed PMMS Target Margin” (Clearwater, Fla., January, 2001), which is incorporated herein by reference. The authors point out that target margin can be specified relative to either current line conditions or to expected worst-case conditions. Because current line conditions can change in the course of a session, as noted above, Moss et al. suggest that the target margin should be based on the expected worst case and put forth standard worst-case parameters for SHDSL service. In practice, however, the worst-case noise varies substantially from one telephone system to another. Even within a given system, the worst-case noise can change over time as the central office adds to or replaces its existing DSL infrastructure. Furthermore, there may be situations in which the actual noise is worse than the standard “worst case,” with the result that the modem will be unable to operate at the rate that was selected according to the worst-case model.
FIG. 1 and 2 schematically shows typical long-loop transmission spectra for Frequency-Domain-Multiplexed Asymmetric DSL (FDM-ADSL) modems and SHDSL modems, respectively. As can be seen in the figures, FDM-ADSL uses separate upstream and downstream transmission bands, while SHDSL uses the same band for both upstream and downstream transmission. The practical reach of DSL modems is often limited to less than 18,000 feet, even for the lowest rates. FDM-ADSL modems in particular are reach-limited, mainly due to two reasons: (1) FDM uses only a part of the frequency band in each transmission direction (because upstream and downstream are transmitted in non-overlapping frequency bands). (2) ADSL operates over Plain Old Telephone Service (POTS), and therefore does not use the lower 25 kHz band. In a long loop, in particular, only the lower frequencies are used, since attenuation is very high at higher frequencies.
Symmetrical services, such as SHDSL, use echo cancellation, and therefore are not limited by point (1) above. Also, since they do not operate over POTS, they are not limited by point (2). Therefore, SHDSL modems, using appropriate echo cancellation schemes, have better reach in laboratory conditions than do FDM-ADSL modems.
In the field, however, SHDSL modems are limited by another factor, which does not limit ADSL: self-NEXT. In the central office, as mentioned above, SHDSL receiver performance is impaired by NEXT induced by the other SHDSL modems, since transmission and reception are performed in the same frequency band. When the central office is loaded with SHDSL services, self-NEXT is the limiting factor in transmission rate performance. DSL standards typically assume that the NEXT noise level encountered by the customer premises equipment (CPE) is close to the noise level at the central office (CO). In many cases, however, NEXT noise at the customer premises is much lower than in the central office. This point is exploited by aspects of the present invention.